Precision optical elements require highly polished surfaces of exacting figure and surface quality. The surfaces demand fabrication in proper geometric relation to each other and, where the elements are to be used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Precision optical elements of glass are customarily produced via one of two complex, multi-step processes. In the first, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogeneous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximating the desired final article. The surface figure and finish of the body at this stage of production, however, are not adequate for image forming optics. The rough article is fine annealed to develop the proper refractive index and the surface figure improved via conventional grinding practices. In the second method, the glass melt is formed into a bulk body which is immediately fine annealed and subsequently cut and ground to articles of a desired configuration.
Both processes are subject to similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular, general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated, polishing techniques which strive to improve surface finish without compromising the surface figure. In the case of aspheric surfaces, this polishing demands highly skilled and expensive hand working. A final finishing operation, viz., edging, is commonly required. Edging insures that the optical and mechanical axis of a spherical lens coincide. Edging, however, does not improve the relationship of misaligned aspheric surfaces, which factor accounts in part for the difficulty experienced in grinding such lenses.
It is quite apparent that direct molding of lenses to the finished state could, in principle, eliminate the grinding, polishing, and edging operations, which are especially difficult and time consuming for aspheric lenses. Indeed, molding processes are utilized for fabricating plastic lenses. Nevertheless, existing plastics suitable for optical applications are available in a limited refractive index and dispersion range only. Furthermore, many plastics scratch easily and are prone to the development of yellowing, haze, and birefringence. The use of abrasion resistant and anti-reflective coatings has not fully solved those failings. Moreover, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics vary substantially with changes in temperature, thereby limiting the temperature interval over which they are useful.
In sum, the overall properties of glass render it generally superior to plastic as an optical material. Conventional hot pressing of glass, however, does not provide the exacting surface figures and surface qualities demanded for image forming optics. The presence of chill wrinkles in the surface and surface figure deviations constitute chronic afflictions. As observed above, similar problems can be encountered in conventional repressing techniques.
Various schemes have been devised to correct those problems, such devices frequently involving isothermal pressing, i.e., utilizing heated molds so that the temperature of the glass being molded will be essentially the same as that of the molds, the use of gaseous environments inert to the glass and mold materials during the pressing operation, and/or the use of materials of specifically defined compositions in the construction of the molds.
For example, U.S. Pat. No. 2,410,616 describes an apparatus and method for molding glass lenses. The molds are capable of being heated and the temperatures thereof controlled within narrow ranges compatible with the glasses being molded. An inert or reducing gas environment (preferably hydrogen) is used in contact with the mold surfaces to inhibit oxidation thereof. The principal inventive disclosure involves the use of a flame curtain (normally burning hydrogen) over the opening of a chamber enclosing the molds to prevent the entrance of air thereinto. No working example specifically illustrating molding process parameters is provided.
U.S. Pat. No. 3,833,347 is also directed to an apparatus and method for press molding glass lenses. Again, the molds are capable of being heated and the temperature thereof closely controlled. An inert gas surrounds the molds to preclude oxidation thereof. The inventive disclosure contemplates the use of mold surfaces composed of glasslike carbon. The use of metal dies was stated to produce lens surfaces which are not suitable for photographic applications. The method involves eight steps: (1) a chunk of glass is placed into a mold; (2) a chamber surrounding the mold is first evacuated and then a reducing gas is introduced therein; (3) the mold temperature is raised to about the softening point of the glass; (4) a load is applied to the mold to shape the glass; (5) the temperature of the mold is reduced to below the transformation range of the glass, while maintaining the load on the mold to prevent distortion of the shaped glass body; (6) the load is removed; (7) the mold is further cooled to about 300.degree. C. to inhibit oxidation of the glasslike carbon; and, (8) the mold is opened. Glass lenses so produced were asserted to be essentially strain free such that no further annealing was necessary.
U.S. Pat. No. 3,844,755 is drawn to an apparatus and method for transfer molding glass lenses. The method contemplates eight steps: (1) placing a gob of optical glass in a transfer chamber fabricated from glass-like carbon; (2) heating the chamber to first evacuate the air therefrom and then introducing a reducing gas therein; (3) heating the chamber to about the softening point of the glass; (4) applying a load to the softened glass to cause it to flow through sprues into mold cavities defined by glasslike carbon surfaces which shape the glass; (5) reducing the temperature of the chamber to below the transformation temperature of the glass, while maintaining the load to prevent distortion of the shaped glass body; (6) removing the load; (7) further cooling the chamber to about 300.degree. C. to inhibit oxidation of the glasslike carbon; and, (8) opening the mold.
U.S. Pat. No. 3,900,328 provides a general description of molding glass lenses utilizing molds fabricated from glasslike carbon. Thus, the patent discloses placing a portion of heat-softened glass into the cavity of a mold prepared from glasslike carbon, applying appropriate amounts of heat and pressure to the mold, while maintaining a non-oxidizing atmosphere in the vicinity of the mold, cooling and opening the mold, and then removing the finished lens therefrom.
U.S. Pat. No. 4,073,654 is concerned with the press forming of optical lenses from hydrated glass. The process comprehends placing granules of hydrated glass into a mold, drawing a vacuum on the mold, heating the mold to a sufficiently high temperature to sinter the granules into an integral shape while the mold is sealed to prevent escape of water vapor therefrom, applying a load to the mold, releasing the load from the mold and opening the mold. Suggested mold materials included glasslike carbon, tungsten carbide, and alloys of tungsten.
U.S. Pat. No. 4,139,677 describes press forming and transfer molding of glass lenses simulating the method of U.S. Pat. Nos. 3,833,347 and 3,844,755 above, but utilizing silicon carbide or silicon nitride as the glass contacting material of the molds, rather than glasslike carbon.
European Patent Application 19342 discloses the isothermal pressing of glass lenses at temperatures above the softening points of the glasses, i.e., at temperatures where the glasses exhibit viscosities of less than 10.sup.7.6 poises. There is no discussion of the manner in which the pressed lenses are cooled to room temperature so it must be assumed that the "conventional" practice was utilized.
In summary, the prior art relating to the isothermal pressing of glass lenses has generally involved pressing at temperatures at or above the softening point of the glass with annealing of the lenses under load within the mold. It is quite apparent that, by its very nature, the process is slow. That is, the pressing cycle involving the time required for inserting the glass into the mold, pressing, annealing in the mold, and removal of the lens from the mold is undesirably long.